Nadal won the match 6-4, 6-3, 6-2 on Court Philippe-Chatrier at Roland Garros. He now has 11 French Open titles, the most ever for a men's or women's player. Nadal broke down into tears after the match, just before the playing of the Spanish national anthem.
"If you told me 7-8 years ago that I would be here at 33-years-old, having this [trophy] ... I would tell you that's something almost impossible, but here we are," Nadal told reporters.
The first set of the match was competitive. Thiem tied the score at 2-2, before going down 5-4. He then unravelled with errors and lost his serve at love. Nadal cruised to a 3-0 lead in the second set and didn't look back.
"I think it was the first time against him here in Roland Garros where it was a fight," Thiem said. "It was a decent match from my side. He was playing very well I think. There is a reason why he won 11 times here. It is definitely one of the best things somebody ever achieved in sports."
The world No. 1 went up 40-0 during Thiem's first service of the third set. Thiem climbed to a 1-0 edge, but Nadal fought to hold a 3-1 advantage.
Nadal, 32, is now 11-0 in his career in the French Open finals. He claimed back-to-back titles at Roland Garros for the eighth time in his career with Sunday's triumph. Nadal won the tournament every year from 2005 through 2008 and from 2010 through 2014.
Thiem, 24, was playing in his first Grand Slam final. The Austrian knocked off Alexander Zverev in the quarterfinals before beating Marco Cecchinato Friday in the semifinals. Nadal climbed to his 11th French Open final after beating Diego Schwartzman and Juan Martin del Porto.
The Spaniard is now 86-2 in his career at Roland Garros. He is three Grand Slam titles behind Roger Federer, who leads the men's field with 20 all-time.